Remove limitation of single root sheet (support flat schematics) (lp:#1688717)

Tracking issue for supporting flat and mixed flat/hierarchical schematics (removing "single root sheet" restriction)

Original report created by Berk Akinci (berka)

I think KiCad should start handling multiple-sheets at each hierarchy
level.

An example of files for one approach could be:
TOP.sch <- Top level of hierarchy
ADC.sch <- Simple single-sheet hierarchical sub design.
PROCESSOR.1.sch <- Sheet 1 of a multi-page hierarchical sub design.
PROCESSOR.2.sch <- Sheet 2...
PROCESSOR.3.sch <- Sheet 3...
LCD.sch <- Another simple single-sheet sub design

In this example, all nets in the PROCESSOR.*.sch files would be treated as
if they existed on the same sheet.

Background:
I've been unsuccessful in finding a good solution to making clean
schematics on eeschema when number of connections increase. I haven't found
this covered on the forums.
At work, I use schematics where a single component with 1000+ pins won't
fit on a single page. (Large components span multiple schematic symbols.)
On KiCad, I can't seem to fit even relatively simple designs.

On KiCad I use hierarchy, but I find that I often can't fit a level of
hierarchy in one page. I've thought hard about how to partition a design
into smaller chunks. I'm assuming I need to break myself of old habits
formed from having used Cadence crap for too long; however, I can't figure
it out. I could make enormous (size) pages, but I'd prefer not to.

For reference: I'm using US B size: 17"x11"
This is because B size is the biggest practically printable size in the
office. Large format printers are not as widely available. Similar on
screen too.  Also, any more content on one page starts making it difficult
to follow the true intent of what is happening. My view of schematics is
like the programming rule of thumb of "any one function should fit in a
screenful". Hierarchy covers some of this, but not all.

For more discussion on this topic, see:
https://forum.kicad.info/t/multiple-sheets-at-same-hierarchy-level

For a public example of types of complicated designs I deal with, see:
http://files.opencompute.org/oc/public.php?service=files&t=e5568c50e71fe52b851b2e386c4c7a27
(from http://www.opencompute.org/wiki/Server/SpecsAndDesigns)
This is just an example.  I have not reviewed this design, and don't
pretend that it is a good example.  It's just a show of complexity that
KiCad should aim to enable.


====== I know this is out of date -- i haven't been actively designing for
a few months. ======
Application: kicad
Version: 4.0.2-stable release build
wxWidgets: Version 3.0.2 (debug,UTF-8,compiler with C++ ABI 1002,GCC
4.2.1,STL containers,compatible with 2.8)
Platform: Mac OS X (Darwin 16.5.0 x86_64), 64 bit, Little endian, wxMac
Boost version: 1.57.0
         USE_WX_GRAPHICS_CONTEXT=OFF
         USE_WX_OVERLAY=ON
         KICAD_SCRIPTING=ON
         KICAD_SCRIPTING_MODULES=ON
         KICAD_SCRIPTING_WXPYTHON=ON
         USE_FP_LIB_TABLE=HARD_CODED_ON
         BUILD_GITHUB_PLUGIN=ON

Original tags: eeschema feature.request